Field effect transistors (FETS) are well known, as are metal oxide semiconductor field effect transistors (MOSFETS). Such transistors are often used for power applications.
Examples of trench field effect transistors suitable for such applications are disclosed in "Comparison of Ultralow Specific On-Resistance UMOSFET Structures . . . " by Syau et al., IEEE Transactions on Electron Devices, Vol. 41, No. 5, May 1994. This document describes the so called INVFET structure 9 of present FIG. 1, which corresponds to FIG. 1(b) of the document. Present FIG. 1 shows only a portion of a single transistor including the polysilicon (polycrystalline silicon) gate electrode 10 which in this case is N type polysilicon which is insulated by a gate oxide layer 12 on its sides and bottom in a trench 14 and insulated on its top side by an oxide layer 18. The trench 14 extends through the N+ doped source region 22 through the P doped base region 24 and into the N+ doped drain region 26. The drain electrode 30 is formed on the underside of the drain region 26 and the source electrode 32 is formed on the top side of the source region 22.
Also described in FIG. 1(a) of that document and shown in present FIG. 2 is the ACCUFET structure 34 which is similar to the INVFET structure 9 except that the base region 24A is N- conductivity rather than P conductivity as in FIG. 1.
The ACCUFET 34 offers the best specific on resistance at the expense of poor blocking capability, while the INVFET 9 offers improved blocking at the expense of increased specific on resistance.
Reference is made to U.S. application Ser. No. 08/415,099, invented by Floyd et al., assigned to Siliconix incorporated, filed Mar. 31, 1995, now U.S. Pat. No. 5,592,005, for a description of the operation of such transistors.
It is the present intent to provide an efficient method for fabricating such transistors of the type described in that patent.